by Donna Leinwand Leger, USA TODAY, USATODAY

by Donna Leinwand Leger, USA TODAY, USATODAY

Leaders of UPS say they've cleared a backlog of holiday packages - but angry customers are demanding refunds because some gifts did not arrive by Christmas.

"We are getting back to normal," said UPS spokeswoman Natalie Godwin.

The worldwide shipper will refund charges for delayed international and air deliveries, Godwin said Friday. Ground shipping is not guaranteed during the peak holiday season so those customers will not get refunds, she said.

Customers continued to complain about delayed shipments and a faulty tracking system that repeatedly promised packages, but didn't deliver.

"Dear @UPS, Is your tracking system a joke?" Scott Persinger of Orinda, Calif. wrote Friday on Twitter. He attached his tracking label that indicated his package was "out for delivery" from San Pablo, Ca., on Dec. 19. He received the same message on Christmas Eve, Thursday and Friday.

Godwin declined to comment on the tracking system.

Martin Almgren of Mansfield, Texas was still waiting on Friday for nine packages, including one shipped via two-day air on Dec. 21. The tracking system shows the package sitting at the Dallas-Fort Worth depot since Dec. 22 and three times the shipper has said the package is out for delivery. Almgren's queries to customer service went unanswered Friday.

Godwin said in an email that all the packages in Dallas-Fort Worth were "out for delivery" on Friday.

UPS says it was overwhelmed by unexpected volume in holiday packages. The company had predicted an 8% increase in package volume over the holidays. The company normally delivers 16 million packages a day.

The company said customers could ship as late as Monday, Dec. 23 by UPS Next Day Air for delivery by Christmas Eve.

To handle the increase, the company hired 55,000 seasonal employees -- the same number as last year, Godwin said.

In a press release Dec. 16, UPS said it expected a surge in packages driven by e-commerce.

"UPS expects to pick up more than 34 million packages today, Monday, Dec. 16, and expects to deliver 29 million packages globally on Tuesday, Dec. 17, a record for a single day at the world's largest package delivery company," the company said. Last year, the company predicted 28 million deliveries on its peak day.

The company planned for 2,388 daily cargo flights in December, 465 more flights than a typical day in November.

Alan Gershenhorn, the chief sales, marketing and strategy officer, said in the press release that UPS "has planned all year to handle the annual increased shipping volume during the holidays."